1. Field of the Invention
This invention relates to a device useful as a master slave clock for transmitting standard time over a telephone network and a telephone network incorporating the device for transmitting and receiving standard time.
2. Description of the Related Art
The requirement of very precise time (of the order of millisecond, microsecond, nanosecond etc.) is limited to users engaged in research in basic science, astronomical studies, space mission, defence activities, etc. But other than these, most users who are interested in time up to one second accuracy would be interested for such service. Number of such users are many including organizations like Railway Station, Airport, some Supermarkets, some government organizations etc. which arrange public display system for time.
Time is one of the several basic quantities from which most physical measurement systems are derived. Others are length, mass and temperature. Unlike other physical quantities it cannot be apprehended by any of the physical senses. For example, we can see distance, feel weight or temperature. But we can know time only through consciousness or observing effects.
When we talk of `time` it may have two meanings: (i) time interval and (ii) time instant. Therefore, the word time requires some additional terms for clarifications. The first one, (i.e. the time interval) is the duration between two events, or in other words, it is the length of time between two events. Time interval may or may not be associated with a specific data. It is of vital importance for synchronization, the very basic need for efficient communication.
The second one, the instant of time, specifies the particular time at which an event has occurred or will occur. Time of a day or date is the most often used term. It is usually presented in a brief form of hour, minute and second, whereas a complete statement of time should also include week, day, month and year. It could also extend to units of smaller than the second going down through milliseconds, microseconds, nanoseconds and picoseconds. We can define time as a continuum that lacks spatial dimensions and in which events occur through irreversible succession from past to future through present.
Accurate and precise means for time information traceable to national and international standards have assumed great importance in recent times. This is due to the demanding uses in various fields. Dissemination means interconnections between the source and the user of time signal. This is like a bridge to make connection between them. Hence to make time accessible to the users the need for time dissemination emerges. There are different types of users depending on the accuracy desired by them. The accuracy desired by them may be of the order of nanoseconds, picoseconds, microseconds, milliseconds and seconds. As the requirement of the order of time accuracy increases the system becomes more complicated and at the same time the cost of the system also increases. The system which can provide highest order of accuracy obviously can serve the purpose of the users requiring lesser order of accuracy. But the use of such a system for the purpose where lower order of accuracy is desired would unnecessarily complicate the system and at the same time the system would be very expensive which is quite undesirable. So, there are several methods of time dissemination having their relative merits and demerits depending mainly on its accuracy, coverage, reliability, ease of use and cost etc. Basic methods of Standard Time and Frequency Signal (STFS) dissemination may be broadly classified into following categories:
Portable Clocks
It is the most accurate and reliable method of time transfer from the time keeping laboratory to the remote user. In this method a portable clock is synchronized at a reference station and carried to desired place in running condition for comparison. To synchronize a remote clock it becomes imperative to fly the clock to reduce travel time. Thus, this method is expensive and is available on demand. This method is accurate to some tens of nanoseconds.
Radio Communication
It is the most important method of STFS dissemination. This dissemination technique is used for comparison of remote clock through radio signal. Previous methods are basically meant for point to point communication. But the radio technique has the primary provision of many users' accessibility simultaneously. The radio communication involves ionospheric propagation up to frequencies of 30 MHz. If the frequency exceeds 30 MHz, the wave propagates in almost a straight line. This is called line of sight propagation (LOS) and forms the basic feature of TV and satellite systems. The satellites are more advantageous than the terrestrial STFS dissemination techniques in terms of global coverage, accuracy of time transfer and propagation feature. Radio communication can be classified as:
A. High Frequency System
B. Very Low Frequency System
C. Satellite System: There are two main categories of STFS dissemination through satellites, e.g.
(a) Geostationary satellites: Geostationary Satellites are normally used to reflect the time signals. There are two basic techniques of time transfer via Geostationary satellite:
(i) One way technique: There are two operational STFS dissemination services in one way mode--one via INSAT originated from National Physical Laboratory, India (A. Sen Gupta, A. K. Hanjura and B. S. Mathur (1991) Satellite broadcasting of time and frequency signals Proc. IEEE, 79, 973) and the other is via GOES satellite originated from NIST, Boulder, USA (R. E. Beehler, D. D. Davis and J. B. Milton, GOES satellite time code dissemination Description and operation, NBS Special Publication 250-300, Jan 1998). The time service via INSAT has the time accuracy capability of ten microseconds. PA1 (ii) Two way technique: Two way technique using MITREX modem has been used for experimental purposes many times to synchronize/compare phasing of seconds pulses of clocks remotely located. This technique amply showed the capability of time comparison accuracy of the order of tens of nanoseconds.
(b) GPS System: Global Positioning System (GPS) (P. Banerjee et. al. (1994) Monitoring of GPS signals at NPL, New Delhi for precise time comparison, IJRSP, 23, 246) is the most accurate world wide time service. This has the capability of few hundred nanoseconds time at any time everywhere.
Wire Communication
Wire communication means to transmit clock time through wire cables. These are used for driving different clock display systems from one central clock for in-house applications. There are two types of wire communication:
Parallel Bit Communication
In this form of wire communication one should dedicate one line for each bit of information, thus requiring N lines for N bits. This is the major disadvantage in parallel communication. Thus dissemination of time signals in parallel form is rather inconvenient for longer distance. But circuit for display system is simple and faster.
Serial Bit Communication
In this case of serial cables there is a pair of wires instead of parallel wires. So bits related to current time data is transmitted sequentially (i.e. one bit at a time is transmitted). This is a slow process and is used in fields where speed is not an important factor. Here the circuit is more involved and thereby it involves more cost. But this is convenient for longer network. These techniques might have been used for local applications but no regular time service using these techniques has been reported yet now.
Most of the available services give time accurate to the order of milliseconds, microseconds submicroseconds etc., thereby making it economically nonviable for large number of users' requirement. None of the above mentioned service can serve the requirement of Standard Time display with a very simple and inexpensive means. The Patent-Search has been conducted to survey the existing patents relating the use of time transfer using telephone network. The critical study of the prior patents indicates that none of them is, somehow, not at all connected to the type of applications the present invention is intended to. A new service which is being planned to be launched by the applicant would be a service through which users would be accessed to Standard Time up to one second accuracy with a very simple equipment connected to normal P&T telephone line. This service would be the first of its kind and would be known as TELECLOCK SERVICE.